The invention relates to a component for detecting electromagnetic radiation, particularly infrared radiation.
Such a component is more particularly intended to be fitted as an optical imaging component, in an infrared camera for example, which operates at ambient temperature in order to constitute what is referred to in the field in question as an “electrical infrared imaging retina”.
In infrared imaging, it may be necessary to place the actual detector inside a chamber where there is a more or less hard vacuum in order to allow correct operation of the detector(s) used and in order to obtain maximum sensitivity to variations in the temperature of observed scenes.
A pressure less than 10−2 millibars is frequently required in order for such a detector to operate satisfactorily. Detectors are therefore encapsulated in a hermetic enclosure inside which the required vacuum or low-pressure atmosphere has been created.
Consequently, when a biometric detector is used, as is often the case in the field of infrared imaging at ambient temperature, it is also necessary to stabilise the temperature of said detector(s) in order to obtain better performance as well as the required sensitivity in terms of accuracy in respect of the temperature of observed scenes.
Such temperature stabilisation is conventionally achieved by using a thermoelectric module, for instance a peltier-type thermoelectric module associated with a PID (Proportional Integral Derivative) controller as well as a thermal sensor placed close to or inside the detector that is to be stabilised and hence inside the enclosure defined by the above-mentioned housing.
An encapsulation package of a bolometric detector in accordance with the prior art is described below in relation to FIG. 1, which is a schematic view.
This essentially comprises a substrate (1) made of a ceramic material or metal or even a combination of both these types of materials. In this case, substrate (1) constitutes the base of the package. It has lateral walls (2) and is hermetically sealed by means of a lid (3) on its upper surface. Lid (3) has a window (4) which is transparent to the radiation to be detected, in this case infrared, and, for instance, transparent to radiation having wavelengths of 8 to 12 μm or between 3 and 5 μm which are common detection wavelengths.
A chamber or enclosure (5) inside which there is a vacuum or low pressure, typically a pressure less than 10−2 millibars, is thus defined. The elements that form this chamber (5) are sealed so that the leak rate in helium is less than 10−12 mbar·l/s.
Inside this chamber, substrate (1) accommodates the actual detector, especially one or more bolometric detectors, located underneath window (4). This or these bolometer(s) (6) is/are associated with an interfacing circuit (7), this assembly or chip being associated with a thermoelectric module (8) attached to a substrate (1), for example by soldering or epoxy bonding. As already stated, this module is intended to ensure control of the temperature of the chip in order, in particular, to act as a reference with regard to the variable analysed by detector (6) and, on this basis, in order to ensure a certain degree of reproducibility of measurements made.
This microbolometer and interfacing circuit assembly (6, 7) is also electrically connected to its environment by means of a wired connection (9) associated with a standard input/output (10) which passes through said substrate (1) and is linked to the electronics of the device in which it is fitted, for example a camera, by means of an interconnection and operating circuit (11).
The heat produced by thermoelectric module (8) is dissipated by means of heatsink (12) placed against the lower surface of substrate (1) and located substantially vertically underneath said module.
In order to maintain the vacuum inside chamber (5), a means capable of absorbing and, generally speaking, pumping the gas molecules released during use of the detecting component is installed inside the chamber, such means being referred to as a “getter”.
This getter (13) is connected to an electric power input (14) which passes through substrate (1), said input also being connected to interconnection circuit (11).
In order to achieve optimum efficiency in terms of thermal control, the thermoelectric module is traditionally placed inside the housing where there is a vacuum or low-pressure atmosphere.
Nevertheless, the possibility of locating this module outside the housing has been considered, thereby making it easier to fabricate the housing and obtain a vacuum inside it, but penalising firstly the efficiency of thermal control in the bolometric detector and secondly the power consumption of said thermoelectric module (see, for instance, Document U.S. Pat. No. 5,914,488 which describes an equivalent principle using a heating module instead of a thermoelectric module).
The disadvantage of incorporating a thermoelectric module and, generally speaking, a thermal stabilisation device inside chamber (5) defined by the housing is essentially the fact that it makes the housing more complex.
In fact, firstly it is necessary to increase the height of the housing, typically by 1.5 to 4.5 mm and, consequently, its overall dimensions for such a device.
Secondly, special-purpose power supply pins must also be provided in order to electrically connect the thermal stabilisation device.
Finally, assembly and outgassing temperatures must be limited to the maximum temperatures which the thermal stabilisation device can withstand.
On the other hand, in the case of a configuration where the stabilisation device or thermal control device is located outside the housing, the power consumption of said device is excessive and the overall dimensions of the entire system, consisting of the housing and the thermal stabilisation device, are larger. This also makes the process of assembling the housing on a PCB more complex (method of connection to PCB, report temperature, etc.).
In addition and regardless where the temperature control device is located, i.e. inside or outside the housing, integrating it with the housing is expensive, in particular due to the cost of manufacturing the actual control device but also because of costs incurred by fitting this device in the housing and costs associated with the specific layout features of the housing in order to accommodate such a control device.